Process for metal coating a hydrogen permeable material



June 23, 1970 WAKEFIELD 3,516,850

PROCESS FOR METAL COATING A HYDROGEN PERMEABLE MATERIAL Filed Sept. 16, 1966 3 Sheets-Sheet :3

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PERCENT CrCl RATIO m GAS PHASE June 23, 1970 wAKEFlELD 3,516,850

PROCESS FOR METAL COATING A HYDROGEN PERMEABLE MATERIAL Filed Sept. 16, 1966 3 Sheets-Sheet 5 1 'UUUU/UUU III/II/l/ United States Patent 3,516,850 PROCESS FOR METAL COATING A HYDROGEN PERMEABLE MATERIAL Gene F. Wakefield, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Sept. 16, 1966, Ser. No. 579,963 Int. Cl. C23c 11/02, 17/00 US. Cl. 117-95 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the deposition of metals upon a substrate by the reduction of the halides of the metals with hydrogen. According to one aspect of the invention, alloy deposits of titanium and chromium, for example, are chemically vapor deposited upon a substrate from controlled mixtures of titanium and chromium halides by reduction with hydrogen. According to another aspect of the invention, one or more metals are deposited upon a surface of a substrate by the reduction of halides of one or more metals with hydrogen diffused through the substrate.

Refractory metals, such as tantalum, molybdenum, columbium (niobium) and tungsten by way of examples, are especially desirable for applications requiring high strength at elevated temperature, particularly where ease of fabrication and ductility are needed. However, before any of these metals can satisfy a wide range of requirements, they must receive an oxidation resistant coating. Titanium and chromium, being metallurgically compatible with refractory metals, form a good base coating which, when alloyed with silicon, will protect them against oxidation.

While many techniques can be employed to prepare coatings of titanium and chromium, several features make the vapor streaming technique of chemical vapor deposition especially desirable. Process parameters may be individually and accurately controlled, allowing high reproducibility. Also, the chemical vapor deposition process possesses the inherent ability to form an overlay coating, that is, a coating which can be applied to a substrate with minimum interaction.

Attempts to co-reduce titanium and chromium halides produced separately and individually introduced into a deposition chamber fail to yield satisfactory co-deposits of titanium and chromium alloys upon a substrate, because of non-uniform mixing of the two halide gas streams and inadequate control of the composition of the reactant gas stream. Furthermore, in the separate production of, for example, the chlorides of the metals by the reaction of hydrogen chloride gas with the individual metals, the unreacted hydrogen chloride gas tends to inhibit the deposition of titanium metal.

Accordingly, objects of the present invention are to provide a method for the simultaneous production of titanium and chromium halides for the co-deposition of titanium and chromium upon a substrate; to provide a method of controlling the composition of the gas stream containing said metal halides; to provide a method for obtaining a uniform mixture of the two halide gas streams;

3,516,850 Patented June 23, 1970 iceto eliminate the presence of hydrogen chloride in the reactant mixture; to provide a method for controlling the rate of reaction and thickness of a chemically vapor deposited metal upon a substrate by diffusing hydrogen through the substrate, and to provide a method for forming uniform coatings of a chemically vapor deposited metal upon a substrate.

In the preferred embodiment of the present invention, controlled uniform mixtures of titanium and chromium halides are obtained by flowing an appropriate titanium halide in an inert gas stream over heated chromium and titanium metals, contained in separate compartments within a generating chamber. The relative concentration of halides obtained from the chamber can be controlled by regulating the amount of titanium halide gas entering each compartment, and by regulating the composition of the metals in each chamber.

The process of the present invention is operationally convenient because, in contrast with the prior art in which the flow rate of hydrogen chloride must be controlled, only the flow of titanium halide must be controlled since no hydrogen chloride is used. The possibility of unreacted hydrogen chloride being present to inhibit the subsequent reduction of the metals is also eliminated which results in higher deposition rates. Furthermore, the method of the present invention does not require that hydroscopic halides of either titanium or chromium be handled in the solid state. Also the equipment and materials contained in the halide generating chamber can be heated under vacuum, if necessary, to remove any absorbed impurity gases without harming the subsequent deposition process. This is not usually possible when halide materials are contained in the reactor itself.

Other objects, features and advantages of the invention will become more readily understood from the following detailed description taken in conjunction with the appended claims and attached drawings in Which:

FIGS. 1 and 2 illustrate suitable apparatus for the generation of controlled mixtures of titanium and chromium halides and the simultaneous co-deposition of titanium and chromium metals from these halides upon a substrate;

FIG. 3 is a plot of the composition of the coating against the composition of the reactant gas phase; and

FIGS. 4 and 5 illustrate suitable apparatus for controlling the rate of reaction and thickness of chemically vapor deposited metals upon a substrate by diffusing hydrogen through the substrate.

Referring now to the figures in detail, FIG. 1 depicts a coating reactor for the reduction of metallic halides with hydrogen. The reactor is comprised of a furnace 1 containing a resistance heater (not shown), a halide inlet 2 at the top of the reactor, a reducing hydrogen inlet 3 at the upper part of one side of the reactor, a substrate 4 to be coated and located directly beneath the furnace 1, a substrate support 5, a substrate heater 14 mounted below the substrate upon the substrate support and an exhaust outlet 6 at the bottom of the reactor.

Mounted within the furnace 1 is a halide generating chamber comprised of a quartz chamber 7 which is divided into two compartments 8 and 9, containing, for example, titanium metal 10 and chromium metal 11, respectively. A divider plate 12 is located at one end of the chamber and an exit 13 is provided at the other end for the impingement of the mixture of gases upon the substrate 4.

Although the method of the present invention is described with reference to titanium and chromium chlorides, it is to be understood that other halides of titanium and chromium may also be utilized in accordance with the principles of the invention by selection of the appropriate apparatus, reactants, flow rates and temperatures.

3 Accordingly, the detailed description given hereinafter is to be taken as exemplary.

Controlled mixtures of titanium trichloride (TiCl and chromium dichloride (CrCl may be obtained by flowing titanium tetrachloride (TiC1 in a stream of inert gas such as argon or helium over heated chromium and titanium metals contained in the compartments 8 and 9 within the chamber 7. The following reactions are believed to occur:

The metals are preferably maintained at a temperature between 750 C. and 900 C.

The preferred manner of controlling the relative concentration of chlorides obtained from the chamber 7 is by regulating the amount of titanium tetrachloride gas (TiCl entering each compartment. This may be accomplished by varying the size and number of holes in the divider plate 12, and by regulating the composition of the metals in each chamber, by which is meant that the same metal, chromium or titanium, is placed in both chambers, that chromium is placed in one chamber and titanium in the other, or that a mixture of titanium and chromium is placed in one chamber and either titanium or chromium is placed in the other. Thus, for example, to deposit an alloy of nearly equal concentration of titanium and chromium, titanium is placed in one compartment 8 and chromium in the other compartment 9. The incoming gaseous titanium tetrachloride from inlet 2, carried in a stream of argon for example, is divided by the plate 12, one part flowing through compartment 8 and the other part flowing through compartment 9. Leaving the individual compartments after severally passing through the metals therewithin, the gases reunite in a common gas stream 13 at the base of the reactor. This control of the ratio of titanium chloride to chromium chloride in the reactant gas stream allows control of the composition of the alloy deposited upon the substrate.

Hydrogen gas in excess of the stoichiometric amount, which passes from inlet 3 downward through the reactor along the outside walls of the chloride generating chamber 7, then mixes into the stream of chlorides in the proximity of the heated substrate 4 to reduce the chlorides and produce pure titanium and chromium according to the following equation:

The pure titanium and chromium then deposit in alloy form upon the substrate 4, the HCl gas and other spent gases exit through exhaust 6. The temperature of deposition, which is preferably maintained from about 1250 C. to about 1375 C., by the substrate heater '14, which is heated by any suitable means (not shown) to the indicated temperature, also affects the ratio of metals in the deposit, higher temperatures favoring titanium reduction; lower temperatures favoring chromium reduction. Coatings formed by the utilization of the above-described chloride source and deposition process are uniform in appearance and thickness.

The relative concentration of halides in the reactant gas stream may also be controlled by the use of a single compartment generating chamber containing alloys of titanium and chromium, as shown in FIG. 2. The composition of the alloy placed in the chamber determines the composition of the alloy deposited upon the substrate.

FIG. 2 depicts a single compartment halide generating chamber comprised of a quartz chamber 21 which contains an alloy of titanium and chromium 22, the rest of the apparatus being the same as shown in FIG. 1. A titanium halide in an inert carrier gas stream is admitted through 4- inlet 23 and reduced by the metals in the alloy to form a stream of reactant gases which exits through outlet 24.

To deposit an alloy of nearly equal concentration of titanium and chromium by way of example, a titaniumchromium alloy 22 composed of 75% titanium and 25% chromium is placed in the chamber 21. The incoming gaseous titanium tetrachloride from inlet 23, carried in a stream of argon for example, is reduced by the titanium and chromium in the alloy to form a mixture of titanium trichloride and chromium dichloride which exits through outlet 24 leading to the deposition site.

In the use of the two compartment halide generating chamber as illustrated in FIG. 1, variation of the proportion of the titanium tetrachloride gas passed through each of the compartments and the selection of the metals placed in either compartment determine the composition of the alloy deposited upon the substrate 4. As described above, to deposit an alloy of nearly equal concentration of titanium and chromium, titanium is placed in one compartment and chromium in the other. Variation of the ratio of the amount of tetrachloride gas passed through the chromium compartment to the amount passed through the titanium compartment further varies the ratio of chromium to titanium in the alloy deposit. Alloys of high chromium metal concentration, for example to chromium and 10 to 5% titanium, have been obtained by placing only chromium metal in the chloride generating chamber. (A high concentration of chromium results because chromium is more readily reduced by the hydrogen gas.) Alloys of high titanium concentration have been produced by substituting a mixture of titanium and chromium metal in the chromium compartment 9, which is then reacted with the titanium tetrachloride. The alloys deposited utilizing this chloride source are about 95% titanium and 5% chromium, which is the result of the preferential formation of titanium chloride over chromium chloride in the gas phase, causing the chromium concentration in the alloy to be correspondingly low.

The following examples further serve to illustrate typical applications of the basic principles of the invention to the simultaneous co-deposition of chromium and titanium within a deposition unit of the type illustrated by FIG.

EXAMPLE I To prepare an alloy deposit of the composition 45% titanium and 55% chromium metal, one compartment of the chloride generating chamber was appropriately filled with titanium, the other with chromium. A helium carrier gas stream containing 0.6 mole percent titanium tetrachloride was passed into the chloride generating chamber at the rate of 7 liters/min. The divider plate contained 10 holes on the titanium side and 2 holes on the chromium side, the holes being 40 mils in diameter. The ratio of titanium trichloride (TiCl to chromium dichloride (CrCl in the reactant gas mixture was 13 to 1. The rate of flow of the reducing hydrogen stream was 7 liters/min. The resulting mixture of gases was reacted at the deposition site at the temperature of 1370 C. The above conditions produced a deposition rate of 1 mg./cm. /min.

EXAMPLE II To prepare an alloy deposit of the composition 93% chromium and 7% titanium metal, both compartments of the chloride generating chamber were appropriately filled with chromium. An argon carrier gas stream containing 1.4 mole percent titanium tetrachloride was passed into the chloride generating chamber at the rate of 5 liters/min. The ratio of titanium trichloride to chromium dichloride in the reactant gas mixture was about 2 to 1. The rate of flow of the reducing hydrogen stream was 4 liters/min. The resulting mixture of gases was reacted at the deposition site at a temperature of 1350 C. The above conditions produced a deposition rate of 2 mg./ cm. /min.

The graph in FIG. 3 expresses the composition of the coating deposit to be expected from any particular ratio of titanium trichloride to chromium dichloride in the reactant gas mixture over a range from near zero to about nineteen.

The coating method of the present invention also offers a practical solution to the problem of corrosion through the application of a corrosion resistant protective layer by chemical vapor deposition to a cheaper, stronger material such as low carbon steel. Application of these materials may be carried out following formation into the final desired shape.

For coating metals which have a high permeability to hydrogen, such as iron, hydrogen may be passed through the iron to form a coating by reaction of the outward diffusing hydrogen with a gaseous halide of one or more metals at the surface of the permeable substrate to be coated. The reaction thus occurs at the interface of the solid containing hydrogen and the gas phase containing the reducible metal halides. For coating the outside of objects such as tubes, the hydrogen is contained on the inside of the tube and the gaseous metal halides are maintained on the exterior of the tube. To coat the interior of the tube, gaseous metal halides are contained within the tube and the hydrogen is introduced from the exterior.

In another embodiment of this diffusion coating technique, a liquid or a fused salt solution containing a reducible metal halide is substituted for the gaseous metal halides. In this case the reaction occurs at the interface of the solid and the liquid containing the reducible metal halide.

Two advantages are obtained by utilizing the diffusion coating method: (1) the thickness of the metal coating is controlled inherently since the thicker portions of the coating tend to produce a slower rate of diffusion. Thus a leveling effect is produced. (2) According to the prevailing view, the state of the absorbed hydrogen at the metal surface is atomic rather than molecular. Atomic hydrogen is a better reducing agent than molecular hydrogen by approximately 50 k-caL/mole, the energy necessary to separate the hydrogen atoms. This permits reaction at temperatures and rates which are impossible with molecular hydrogen, such as the reduction of titanium chloride at 700 to 800 C. However, applicant does not wish to be bound by the accuracy of the above stated prevailing view.

FIG. 4 illustrates suitable apparatus for practicing an embodiment of the diffusion coating method described above, wherein the interior or exterior surface of a hydrogen permeable tube may be coated by the reaction of hydrogen and a gaseous metal halide by Way of example. The coating reactor comprises a furnace 41 provided with heating coils 42 which maintain the furnace at the appropriate deposition temperature. Inlets 43 and 44 are located at the top of the reactor. The substrate to be coated consists of a tube 45 which is attached to inlet 44 within the chamber 46. Exhaust outlet 47 at the end of tube 45 and exhaust outlet 48 at the bottom of the chamber 46 are for the exit of the spent gases. To coat the outside of the tube 45, hydrogen is introduced through inlet 44 and gaseous metal halides through inlet 43. To coat the inside of the tube 45, gaseous metal halides are introduced through the inlet 44 and hydrogen through inlet 43.

The following example illustrates a typical application of the basic principles of the diffusion coating method of the invention within a deposition unit of the type illustrated by FIG. 4.

EXAMPLE III The reactor was first brought to an operating temperature of 1000 C. An argon carrier gas stream containing 1.7 atomic percent titanium trichloride (TiCl was admitted through inlet 44 and passed through a steel tube 45 having a wall thickness of 32 mils. Hydrogen gas was admitted through inlet 43 and allowed to purge the chamber 46 of atmospheric gases. Thereafter, the exhaust valve 48 was closed and the hydrogen gas was maintained in chamber 46 at a pressure of 25 p.s.i. An operating time of 30 minutes produced a coating of titanium on the interior of the tube on the order of 1 mil in thickness.

FIG. 5 illustrates suitable apparatus for practicing another embodiment of the diffusion coating method of the invention, wherein a flat hydrogen-permeable sheet may be coated by the reaction of hydrogen and a reducible metal halide contained in a fused salt solution. FIG. 5 depicts a flat iron plate 51 to be coated on its upper surface, mounted between a hydrogen chamber 52 provided with heating coils 53 and a fused salt chamber 54 also provided with heating coils 55. These coils are shown separate but they need not be. Hydrogen chamber 52 is further provided with an inlet 56 near the top of one side of said chamber and an outlet 57 near the bottom of the other side.

The following example illustrates a typical application of the basic principles of the diffusion coating method of the invention within a deposition unit of the type illustrated by FIG. 5.

EXAMPLE IV To coat a flat iron plate 5 mils thick, a mixture of 5 to 10 atomic percent chromium trichloride (CrCl in a potassium chloride, lithium chloride liquid eutectic was maintained in chamber 54. Hydrogen gas was admitted through inlet 56 and allowed to purge the chamber 52 of atmospheric gases. Thereafter, the exhaust valve 57 was closed and the hydrogen gas maintained in chamber 52 at a pressure of 25 p.s.i. The apparatus was operated at 850 C. for 30 minutes to produce a crystalline deposit of chromium 0.5 mil thick on the upper surface of the flat iron plate.

Thus it may be seen that the invention supplies a method for the simultaneous production of titanium and chromium halides for the co-deposition of titanium and chromium upon a substrate which advantageously, provides a control of the composition of the gas stream containing said metal halides, and results in uniform mixtures of the two halide gas streams. A further advantage of the invention is the elimination of hydrogen chloride from the reactant mixture, thus favoring the deposition of titanium. The invention also provides a method for controlling the rate of reaction and thickness of a chemically vapor deposited metal upon a substrate by the diffusion of hydrogen through the substrate, thereby forming uniform coatings.

It is to be understood that the above-described embodiments of the invention are merely illustrative of its principles. The apparatus shown and described may be modified to coat substrates of varying configurations using reducible metal halides contained in either a gaseous stream or a fused salt solution. Various other modifications may be devised by those skilled in the art to fit the character of the substrate to be coated Without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. A method of controlling the rate of reaction and thickness of a metal coating deposited upon a hydrogenpermeable material by controlling the rate at which hydrogen is admitted to the reaction, comprising the steps of:

(a) heating said hydrogen-permeable material;

(b) diffusing hydrogen from a first side of said material through said material; and

(c) maintaining a mixture of halides of one or more metals on a second side of said material, whereby said hydrogen reacts with said mixture of halides at the interface of the solid containing the hydrogen and the phase containing the reducible metal halide 7 8 mixture, thereby forming a metal coating at said ininterface of the solid containing the hydrogen and terface until said coating reaches a thickness which the phase containing the reducible metal halide mixis essentially impermeable to hydrogen. ture, thereby forming a metal coating on one surface 2. The method of claim 1 wherein said halides are conof said tube at said interface until said coating tained in aliquid. 5 reaches a thickness which is essentially impermeable 3. The method of claim 2, wherein said liquid is a poto hydrogen. tassium chloride-lithium chloride liquid eutectic. References Cited 4. The method of claim 10 wherein said halides are UNITED STATES PATENTS gaseous.

5. A method of controlling the rate of reaction and 10 2,887,407 5/1959 KOCH 117' 107-2 thickness of a metal coating deposited upon one surface 3,072,498 1/1963 Knowles et 117'113 of a hydrogen-permeable tube by controlling the rate at 3,147,154 9/1964 et 117 113 X which hydrogen is admitted to the reaction, comprising 3,351,487 11/1967 Levme at 117 160 X the steps of: 3,373,018 3/1968 Oxley et a1. l17107.2 X

(a) heating said tube;

(b) diffusing hydrogen through the wall of said tube 15 ANDREW GOLIAN Pnmary Exammer from a first side thereof; and U S Cl X R (c) maintaining a mixture of halides of one or more metals on a second side of said tube whereby said 117-94, 107.2, 113, 160 hydrogen reacts with said mixture of halides at the 20 

